DC Circuits I
Physics 2415 Lecture 12
Michael Fowler, UVa
Today’s Topics
•
•
•
•
•
Mention of AC
Semiconductors and superconductors
Battery emf, internal resistance
Series and parallel resistances
Kirchhoff’s rules
AC and DC
• Batteries provide direct current, DC: it always
flows in the same direction.
• Almost all electric generators produce a
voltage of sine wave form:
V  V 0 sin 2 ft  V 0 sin  t
• This drives an alternating current, AC,
I 
V 0 sin  t
R
 I 0 sin  t
and power
P  V I  I R  I 0 R sin  t   V 0 / R  sin  t
2
2
2
2
2
AC Average Power and rms Values
• The AC power P   V 02 / R  sin 2  t varies
rapidly ( = 2f, f = 60 Hz here), what is
significant for most uses is the average power.
value of sin t
• The average value of sin2t is ½. average
must equal average value
2
• Define Vrms by V rms

V
2
• Then the average power
 V0 /
2
of cos2t. and remember
sin2t + cos2t = 1
P  V rm s / R
2
The standard 120V AC power is Vrms = 120V.
So the maximum voltage V0 on a 120V line is 120x2 = 170V!
Sometimes DC is used for a Single Long
Line
• This 3 gigawatt DC line
(enough for 2 to 3 million
households) transmits
hydropower from the
Columbia river to Los
Angeles.
• At these distances, it gets
tricky synchronizing the
phase of AC power.
Semiconductors
• In the Bohr model of the hydrogen atom, an electron
circles around a proton.
• An n-type semiconductor is a dielectric insulator which
has been doped—atoms having one more electron
than the insulator atoms are scattered into it.
• The extra electron circles the dopant atom, but is
loosely bound because the dielectric shields the electric
field. As the temperature is raised, these electrons
break away from their atoms, and become available to
conduct electricity.
• Bottom Line: Conductivity increases with temperature.
Superconductors
• A superconductor has exactly zero resistivity.
• In 1911, mercury was discovered to
superconduct (R = 0) when cooled below 4K.
• Superconducting magnets are widely used, in
MRI machines, etc.
• There are now materials superconducting
above the boiling point of liquid nitrogen,
making long distance transmission lines
feasible.
• Superconductivity is a quantum phenomenon.
Battery emf E
• At the terminals inside a battery, a precise
voltage is generated by the particular chemical
energy exchanges taking place (electron
capture or donation by molecules at the
trerminals).
• This voltage is called the electromotive force
(evne though it’s a potential energy, it does
drive the current around a circuit), and
denoted by emf or E.
The emf E and Internal Resistance
• This chemically generated voltage E also has to
push the current through the battery itself.
• The battery has an internal resistance, usually
denoted by r, so for a current I in the circuit, the
battery supplies to the outside world a terminal
voltage
• V=
E - Ir
• (This is usually a small effect and can be neglected.)
Resistances in Series
• A battery voltage V pumps a steady current I
a
•
.
through 3 resistances in series, as shown.
R1
• Think of the battery as a pump, raising the
potential of charge, which then drops in the
b
R’s, like a series of waterfalls a b c d.
R2
• From Ohm’s Law, the potential drops are:
c
Vab = IR1, Vbc = IR2, Vcd = IR3.
R3
• So the total drop V = Vad = Vab + Vbc + Vcd =
IR1 + IR2 + IR3 = IR,
d
where the total resistance R = R1 + R2 + R3
I
V
Resistances in Parallel
• .
• (Convention: lines without zigzag
I
represent wires of negligible resistance.)
I1
• This means all three of the resistances
shown have the same voltage V between
I2
their ends.
• So V = I1R1 = I2R2 = I3R3
I3
• The total resistance is defined by V = IR.
V
V
V
V
• Now I  I 1  I 2  I 3     ,
R1
giving
1
R

1
R1

1
R2

R2
1
R3
R3
R
V
R1
R2
R3
Clicker Question
• Which has the greater resistance,
A. A 120V 60W bulb?
B. A 120V 30W bulb?
Clicker Question
• Which has the greater resistance,
A. A 120V 60W bulb?
B. A 120V 30W bulb?
Remember power P = VI = V 2/R. V is the
same for both, so lower R means higher
power.
Clicker Question
• If a 60W bulb and a 100W bulb are connected
in series to a 120V supply, which will be
brighter?
A. The 60W bulb
B. The 100W bulb
C. They’ll be equally bright
Clicker Answer
• If a 60W bulb and a 100W bulb are connected
in series to a 120V supply, which will be
brighter?
A. The 60W bulb
B. The 100W bulb
C. They’ll be equally bright
D. The 60W bulb has greater R, so more voltage
drop—and power = VI, they have the same I.
Remember…
• Resistances in series all carry the same current
• Resistances in parallel all have the same voltage
drop
• Put this together with Ohm’s law for each
resistance.
General Circuits: Kirchhoff’s Rules
• Junction Rule: when several wires meet at a
point, the total current flowing into the point
must equal the total current flowing out. Charge
cannot disappear, or pile up at a point.
• Loop Rule: the total potential (voltage) change
on following wires around a loop to your starting
point must be zero.
• (The loop rule is equivalent to saying that if you follow
some random path on a hillside, and get back eventually
to your starting point, your net change in height above
sea level is zero.)
Clicker Question
d
r
b
a
All lines have resistance 1
except dc, which has resistance r.
If a voltage v is applied from
a to b, which way does
current flow in dc?
c
A. From d to c
B. From c to d
C. There is no current
Clicker Answer
d
b
a
All lines have resistance 1
except dc, which has resistance r.
If a voltage v is applied from
a to b, which way does
current flow in dc?
c
A. From d to c
B. From c to d
C. There is no current
There is no current because the
situation is completely symmetrical:
symmetry can sometimes simplify
circuit analysis.
Problem
d
All lines have resistance 1
except dc, which has resistance r.
r
b
a
c
If now a voltage 10V is applied from
a to c, what is the total current flow?
.